Japanese Registered Utility Model No. 2545621 (“PTL 1”) discloses a spark plug including a capsule that forms an ignition chamber. The spark plug is configured such that the ignition chamber has a narrowest portion in an electrode gap or in the vicinity thereof and such that the diameter of the ignition chamber decreases from a first plane positioned below the narrowest portion toward the bottom. The capsule forming the ignition chamber includes four side orifices and one bottom orifice.
In the spark plug, when a spark is generated in the electrode gap, the spark ignites an inflammable air-fuel mixture in the ignition chamber. The ignited air-fuel mixture expands and injects flame through the side orifices and the bottom orifice. These flame jets ignite an inflammable air-fuel mixture in a main combustion chamber.
According to the technology disclosed in PTL 1, however, even when an effect of increasing the temperature of a combustion gas in a pre-chamber by narrowing a portion of the ignition chamber is exerted, the diameter of the ignition chamber is decreased below the narrowest portion and therefore, there is a possibility of a pressure loss occurring at the decreased-diameter portion. In addition, according to the technology disclosed in PTL 1, since the capsule includes the bottom orifice, the length of flame injected from the side orifices may decrease. As a result, there is a possibility that high-temperature flame may not reach the end of the combustion chamber and the combustion speed of an engine may decrease, which is a problem.